I don't mean to be a dick but I find that extremely hard to believe. I've got a superbike (GSXR 1000) and an Audi R8 V10 (which yes is heavier than an MX5 but has 520 bhp) and the bike is A LOT faster. They do 0 - 60 in 3 seconds. Are you seriously telling me your MX5 could do anything close to that? I've also had a Lotus Elise which is even lighter than an MX5 and that had 180 bhp and was slow as hell compared to a superbike, so I'm not quite seeing how an MX5 with only 100 bhp more could begin to keep up with a superbike up to 160 mph. No offence and maybe I've misunderstood something you said, but just seems a kind of ridiculous claim if I've understood it correctly
Pretty simply really, same reason most supercars aren't that fast compared to tuned 'normal' cars.
Audi R8 V10 is 520bhp and 390lbft, is 4wd so loses at least 100bhp/100lbft with transmission losses, is geared to the moon (way beyond 200mph), and weighs about 1700kg= About 245bhp per ton and 170lbft per ton at wheels.
Shitbox stripped bare MX5 pushing ~270bhp and the same in torque, weighs 1000kg tops (prob way less as it was just a shell), geared to 160mph on the limiter in 5th, and RWD means ~25bhp/25lbft transmission loss= About 245bhp per ton and 245lbft per ton at wheels. Not that fast for my liking, but still fast.
If your R8 was geared down to top out at 160odd too, I'd expect it to keep up with a superbike to 160odd, but long gears kill acceleration, hence how Rallycross cars do 0-100mph in just over 4.5sec despite weighing over 1300kg.
To 160 from a 30mph roll (I had no wheelspin, tarmac rally slicks), I lost by I think it was 3 lengths or so to 160 to a friend on his R1 if that helps, most of that at 140+.
From a standing start though, no chance. It's nothing that special, I've had faster cars, but it was fun for what it was as it weighed cock all.
I'm surprised your R8 V10 is that much slower than your bike after 100mph+ to be honest. Have you tried a side by side race?